The invention relates to circuitry for radio frequency (RF) transmitters and in particular to amplifier circuitry providing signal combining integral to the power amplification.
Typically, power combiners are used in RF transmitters to combine the output signals of parallel power amplifiers into one high power RF output signal for wireless transmission. In these known transmitter structures the signals are first amplified by the power amplifiers and then they are combined by a power combiner to produce a combined amplified signal for transmission. Depending upon the circuit architecture and signal format used, however, it becomes necessary to make trade-offs between reducing power losses and achieving isolation between input signals of the combiner.
The need for efficiency is a particularly important design factor for the highly integrated requirements of transceivers used for wireless local area networks (LANs) and employing modulation formats such as OFDM (Orthogonal Frequency Division Multiplex). Moreover, the inventor has developed signal modulation methods, using OFDM signal format, whereby information signals are deconstructed into independent component signals, which are then more efficiently processed and modulated than the original information signals, for up-conversion, amplification and combination prior to transmission. These independent modulated signals present additional challenges to achieving efficiency at the amplification/combination stages of the transmitter when using the conventional model of amplification followed by combining due to inherent loss and isolation limitations of the known power amplifiers and combiners.
Non-reciprocal combiners are non-economic for applications such as low cost wireless and, instead, reciprocal combiners, realized as either four-port or three-port structures, are available for use in such applications. Four-port combiners provide an advantage of isolation between the individual inputs (which means that the output impedances of the amplifier stages do not load each other) but where the signals being amplified are non-identical (i.e. statistically independent) an inherent loss of 3 dB results (this loss disappears where the signals are identical due to resonance). Thus, four-port combiners are generally only suitable for use where the signals being amplified are identical.
A three-port combiner, also known as a trifilar, is able to provide a degree of isolation between its individual inputs, depending on the output impedance of the amplifiers feeding it as well as the load impedance connected to the combiner""s output. If the output impedances of the individual amplifiers and the output loading impedance of the combiner are the same, then no isolation is provided, resulting in an inherent loss of 3 dB. On the other hand, if the output impedances of the amplifiers are small in comparison with the output loading impedance of the combiner, then the inherent loss diminishes, and approaches 0 dB for 0 ohms output impedance.
The many classes of power amplifiers can be broadly sorted into two classifications; linear and switched-mode. Linear amplifiers provide an output-impedance resulting from the bias condition and load line for the active device (in the usual case the active device being a transistor). In practice, this output impedance is typically in the range of 5 to 50 ohms. As a result, only limited isolation is achievable when using a three-port combiner (trifilars) to combine the outputs of two linear amplifiers. A conventional switched-mode power amplifier (class D or class E) consists of an input component of active (switching) devices, a central transformer component and an output component consisting of a resonator. It is impractical to apply the output signals of separate switched-mode amplifiers to a trifilar to combine them because of the cost and space requirements (and resulting inefficiency) associated with the multiple transformer windings required for such a design.
There exists a need for new and efficient means to achieve power amplification and combining of modulated signals in transmitters.
A switched-mode power amplifier is configured for performing power amplification of a plurality of signals input thereto (i.e. analog phase modulated signals), and integrally summing (combining) those signals. Conceptually, this is achieved by replacing the center-tapped input winding component of the transformer within a conventional balanced-type transformer-coupled voltage switching amplifier with separate input winding components, one for each input signal, in similar manner to the configuration of the input components of a conventional three-port combiner (trifilar). Accordingly, the input winding of the amplifier""s transformer is comprised of a plurality of series-coupled windings, one for each of the plurality of input components/signals.
In accordance with the invention there is provided a switched-mode power amplifier configured for integrally amplifying and summing a plurality of signals input thereto. The amplifier comprises an input component for each of the plurality of input signals, an output resonator component and a transformer component there between. The transformer component comprises a plurality of series-coupled input windings and an output winding. Each input component comprises an input winding and a plurality of active devices wherein the active devices are configured to be alternately switched by the input signal for that input component so as to present an amplified signal corresponding to the input signal at the input winding. Each input component provides a low output impedance. As a result of the series-coupled amplified signals on the input windings of the transformer, the output winding presents a summation signal corresponding to a summation of those amplified signals.
In one preferred embodiment the input component of the amplifier of the invention comprises two active devices and the input winding is a center-tapped winding the center tap of which is connected to a voltage rail, each terminal end of the winding being fed by one of the active devices.
Advantageously, the active devices alternately switch between a low output impedance and a high output impedance whereby the resulting parallel output impedance is low. Consequently, the series-coupled center-tapped windings of the plurality of input components constitutes a series connection of low output impedance sources applied to the amplifier""s resonator and load. This, in turn, provides a high level of isolation between the amplifier input components and results in a low level of loss.
In another preferred embodiment the input component of the amplifier of the invention comprises two coupled pairs of active devices arranged in bridge configuration with the input winding, wherein each pair is alternately switched, one active device of each pair being connected to a voltage rail and the other active device of each pair being connected to ground, the input winding coupling together each pair of active devices.
Also in accordance with the invention there is provided a method for amplifying and summing a plurality of input signals to produce a single amplified, summation signal for input to a resonator component. Each input signal is amplified by a separate amplifier input component having a low output impedance to produce an amplified signal corresponding to the input signal within a winding of the input component. To perform the amplifying the input signal is applied to active devices of the input component to cause alternate switching of the active devices. The windings of the input components are coupled in series to provide an effective input winding for a transformer component. The summation signal is thereby presented within an output winding of the transformer configured for output to the resonator component.